The safe disposal of waste is an ever growing, worldwide concern; and landfill technology has been developed to provide for the safe and economical disposal of solid waste. A typical modern landfill comprises a waste-receiving pit excavated into the ground and lined with a moisture impervious material to isolate the waste from groundwater. The landfill generally includes a collection system for withdrawing liquid, commonly referred to as leachate, from the waste pit. The leachate is comprised of moisture which enters the pit as well as moisture generated within the waste itself. When full, the landfill pit is closed by a moisture impervious cap which is typically configured so as to drain water away from the pit. The cap is usually pierced by one or more vents which establish a passage for the flow of gases from the landfill. These gases comprise methane and other by-products of waste decomposition and they are typically burned or otherwise chemically neutralized.
The cap is an important part of the landfill since it serves to isolate the waste in the landfill from the exterior environment. The cap prevents the exit of pathogens, toxins and odors from the landfill and prevents access of vermin to the waste. The cap also serves a very important function in preventing access of water to the interior of the landfill; this is necessary to minimize the amount of leachate in the landfill and to preserve the integrity of the pit lining.
Prior art landfill caps typically comprise a layer of about two feet in thickness of a low hydraulic conductivity material such as clay or soil with a high clay content. A drainage layer of sand, gravel or other permeable material is disposed atop the low hydraulic conductivity layer and a layer of topsoil, planted with vegetation covers the structure. The cap is typically shaped so as to facilitate drainage of rainwater away from the landfill; in operation, rainwater and the like passing through the layer of soil and vegetation is stopped by the low hydraulic conductivity layer and flows away from the landfill through the permeable layer.
Problems have been encountered with prior an landfill cap structures as a result of cracking of the clay based layer. Cracking can result from settling of the layer as waste in the landfill compacts. Additionally, freezing and thawing soil can crack and heave the clay layer. Also, desiccation of the clay layer can produce cracks. Problems are also encountered at the points where gas vents and leachate pipes penetrate the layer since it is difficult to obtain a good seal thereabout, and the site of penetration can become a crack initiator. This problem is particularly severe when a new opening must be made through an established cap. In addition to the foregoing, fabrication of clay based caps can be difficult, since care must be taken to utilize soils having a proper moisture content if an impervious seal is to be obtained. Also, movement of equipment across the layer while it is being formed is likely to create cracks, particularly when weather conditions are very dry.
Because of these acknowledged problems with clay based capping layers, the prior art has, in some instances, further included polymeric sheets in landfill caps. These sheets are typically made of low density polyethylene or polyvinyl chloride and are used in combination with the clay based layer. Typically the polymeric sheet is placed atop the clay layer prior to placement of the drainage layer. While the polymeric sheet material improves the performance of the cap structure, new problems are encountered in its use. The polymeric material is easily punctured during installation, or by waste in the landfill or rocks, branches and the like encountered in the ambient environment. Additionally, the polymeric sheet is prone to tear as a result of strain caused by settling of the landfill or friction of the sheet against the clay based layer caused by the overburden of the drainage layer and vegetation layer, particularly at the highly sloped portions of the cap structure. The polymeric sheet does not solve the problems caused by feed throughs since it is not capable of effecting a tight seal to pipes, tubes and the like. Also, polymeric sheet material is fairly difficult to install over large surface areas, and care must be taken to insure that installation equipment does not puncture or tear the sheet. If smaller area sheets are utilized or the landfill is large, they must be joined together, and the formation of strong, moisture tight seams in large area polymerized sheet material is quite difficult to achieve and expensive.
In view of the foregoing, it will be appreciated that there is a need for a landfill cap structure which is capable of reliably sealing a landfill against entry of ambient moisture for relatively long periods of time. The cap structure should be stable to a wide range of environmental conditions and should be resistant to damage from freeze/thaw cycles, ground subsidence and the like. In addition, the cap structure should provide for a moisture tight seal to conduits, cables and other such elements which pass through the cap. Finally, the cap structure should be low in cost and easy to install.
As will be explained in greater detail hereinbelow, the present invention provides a landfill cap structure which includes a seamless, monolithic, conformal body of elastomeric material. The body of elastomeric material is preferably applied by a spray process and provides a moisture-tight seal to subjacent layers as well as to conduits, vents and the like which pass therethrough. The elastomeric layer of the present invention is stable to the ambient atmosphere as well as to the chemical environment of the landfill. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.